Voltage reference circuits are widely used in microelectronic integrated circuits, to provide a voltage reference that can be independent of temperature and/or power supply variations. One widely used voltage reference circuit is a bandgap voltage reference, which produces an output voltage of about 1.25 V, close to the theoretical bandgap of silicon at 0 K.
A simple bandgap circuit can utilize the voltage difference between two diodes, to generate a Proportional To Absolute Temperature (PTAT) current in a first resistor. This current may be used to generate a voltage in a second resistor. This voltage, in turn, is added to the voltage of one of the diodes (or a third diode). The voltage across a diode operated with PTAT current is Complementary To Absolute Temperature (CTAT), i.e., it reduces with increasing temperature. If the PTAT and CTAT characteristics are complementary, they can cancel out, to produce a resulting voltage that is independent of temperature. In other words, Constant With Temperature (CWT) output may be provided.
As the integration density of integrated circuits continues to increase, and the power supply voltages continue to decrease, it has become desirable to provide bandgap voltage reference circuits that do not occupy excessive integrated circuit real estate, and can also consume very little power. In fact, nano-Ampere current draws may be desirable. As a result, voltage reference circuits and circuits that are based upon voltage reference circuits have increasingly used insulated gate Field Effect Transistors (FETs), commonly referred to as MOSFET devices or CMOS devices.
For example, a publication entitled “A Low-Voltage CMOS Bandgap Reference” to Vittoz et al., IEEE Journal of Solid-State Circuits, Vol. SC-14, No. 3, June 1979, pp. 573-577, illustrates at FIG. 7 a basic cell of a PTAT voltage source, wherein two transistors are serially connected with a common gate connection, and operate in weak inversion. A stack of these elementary PTAT cells is illustrated in FIG. 9, to provide a desired output voltage for the stack of elementary PTAT cells. Other voltage reference circuits are described in a publication by Sansen et al. entitled “A CMOS Temperature-Compensated Current Reference”, IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988, pp. 821-824, and in U.S. Pat. No. 5,798,669 to Klughart, entitled “Temperature Compensated Nanopower Voltage/Current Reference”. A summary of the design of bandgap reference circuits may be found in “The Design of Band-Gap Reference Circuits Trials and Tribulations” by Robert Pease, IEEE 1990 Bipolar Circuits and Technology Meeting, Minneapolis, Minn., Sep. 17-18, 1990.